The present invention relates to an accurate signal generating circuit which can be manufactured in the form of an integrated circuit.
A conventional signal generating circuit of the same general type to which the invention pertains is shown in FIG. 1A. In FIG. 1A, reference numeral 1 designates a timing pulse signal generating circuit; 2, a comparison circuit; R.sub.1 and R.sub.2, comparison voltage generating resistors; V.sub.1, a comparison voltage; Q.sub.1, a switching transistor; R, a timing resistor which is externally connected to the circuit; and C, a timing capacitor which is also externally connected to the circuit.
When the timing signal generating circuit produces a pulse of a signal S.sub.1, the switching transistor Q.sub.1 is turned on and the voltage at the circuit point S.sub.2 becomes the collector saturation voltage V.sub.CES of the transistor Q.sub.1, as shown in FIG. 1B. When the signal S.sub.1 returns to the low level, the switching transistor Q.sub.1 is rendered nonconductive. As a result, a charging current flows through the capacitor C via the resistor R. In this case, the voltage E at the circuit point S.sub.2 can be represented by the following expression: EQU E=V.sub.cc (1-e.sup.-t/RC)+V.sub.CES.
The period of time T required for establishing E=V.sub.1 is: EQU T=RC log.sub.e (1-(V.sub.1 -V.sub.CES)/V.sub.cc).
When E becomes equal to V.sub.1, the state of the comparison circuit 2 is changed and the comparison circuit provides an active output.
In the conventional signal generating circuit designed as described above, the period of time T required for the voltage at the circuit point S.sub.2 to become equal to the comparison voltage includes terms of the supply voltage V.sub.cc and the collector saturation voltage V.sub.CES. Accordingly, the period of time T is affected by variations of the supply voltage V.sub.cc and the variations in the collector saturation voltage V.sub.CES. Temperature changes also affect T. Therefore, it is impossible to determine the period of time T with a high accuracy. This is especially difficult in a low voltage circuit operating from a low supply voltage.
An object of the invention is to overcome the above-described difficulties accompanying a conventional signal generating circuit. More specifically, an object of the invention is to provide an accurate signal generating circuit which is not affected by variations of the supply voltage and temperature or by variations in characteristics among transistors, which has a simple circuit arrangement, and is suitable for manufacturing in the form of an integrated circuit.